Metal pickling solutions and methods



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METAL PICKLING SOLUTIONS AND METHODS Howard B. Bomberger, East Liverpool, Ohio, and Milton B. Vordahl, Beaver, Pa., assignors, by mesne assignments, to Crucible Steel Company of America, Flemington, N.J., a corporation of New Jersey Original application June10, 1954, Ser. No. 435,755. gigigeg and this application Nov. 16, 1956, Ser. No.

4 Claims. (Cl. 134-3) a and Thus, if a metal is picked in a reducing acid solution' such as hydrogen fluoride, hydrogen ions are reduced and atomic hydrogen diffuses into the metal. With metals such as titanium, this results in a serious loss 'in ductility. Thus However, if a cation or an oxidizing agent is present which is more readily reduced than hydrogen ions, no hydrogen is evolved or absorbed by the metal. For e xample, on pickling titanium in an acid solution containing fluoride ions and hydrogen peroxide, the peroxide is reduced to water along with the formation of a titanium for heat treating or for hot rolling or forging operations or the like. Removal of this embrittled surface layer, as by acid pickling, improves ductility, particularly bend ductility. p p I, Most acids do not, however, readily attack corrosion resistant metals like titanium, zirconium and the majority of their alloys in which titanium or zirconium is the major constituent. And amonghthose which do, the majority hydrogen-embrittled and/or deeply pit the surfaces of the metal.

In accordance with the present invention, we have discovered that notable exceptions to the above, are appropriate concentrations of hydrofluoric acid or equivalent amounts of fluoride salts when employed in aqueous pickling solutions containing strong oxidants, such as nitric acid or hydrogen peroxide or both in critically controlled amounts.

The present method of pickling titanium andtitanium alloys is to use the conventional steel plant pickling bath comprising an aqueous solution containing about 10% by weight of nitric acid and 2% of hydrofluoric acid and 1 maintained at about 160 F. forremoving from about 0.002" to 0.004" of metal from the surface in about 2 to 20 minutes.

The major disadvantages of this practice are: (1) Substantial amounts of hydrogen are absorbed by the metal resulting in serious loss of ductility; (2) copious quantities of the hazardous nitrogen dioxide gas are evolved which sometimes exceeds available ventilating capacity; and (3) the bath is diflicult to control. Rapid depletion results in dangerously low concentrations of nitric acid.

The dissolution of metals in aqueous pickling solutions is basically an electrochemical phenomenon resulting in the formation of metallic ions. This is known as an oxidation reaction, e.g., in the case of titanium in a reducing acid,

A necessary requirement for this reaction to proceed is the simultaneous reduction of either cations or an oxidizing agenta reduction reaction. In a reducing solution, hydrogen ions are reduced, resulting in the formation of atomic and molecular hydrogen, i.e.,

(2) H +e- H fluoride complex.

And, on pickling titanium in a nitric acid solution con 7 taining-fluoride ions, the nitric acid is reduced, forming a titanium fluoride complex, water and colorless nitric:

oxide (NO). The probable reaction is: V ('6) 3Ti+4HNO +12HF 3TiF +8H O+4NO The nitric oxide is immediately oxidized to the brown;.

poisonous nitrogen dioxide on exposure to air, i.e.,

t 2N 2N0. Thedioxide' is very soluble in, water, forming nitric acid and nitric oxide, which can-also be oxidized to the;

dioxide, thus forming nitric acid, e.g.,

If the concentration of the oxidant is too low, the reduction of hydrogen ions may occur along with the reduction of the oxidizing agent. The rate of a given such reaction is a function of concentration and mobility of reactants and products, changes of free energy and other factors. In this case there is some absorption and evolution of hydrogen. This condition is illustrated in the graph of the accompanying drawing in which the percent hydrogen absorbed on pickling 0.004" from Tl-e 7Mn sheet at 150 F. is plotted as ordinates against the nitric acid content as abscissae for an aqueouspickling solution containing 2% hydrofluoric acid in addition to nitric acid. It will be noted therefrom, that below con-. centrations of 15% HNO substantial amounts of hydrogen are absorbed.

In accordance with the present invention, the most desirable pickles are found to consist of aqueous solutions of strong water-soluble oxidants and hydrofluoric acid in concentrations of about /2 to 10% of the fluoride ion, i.e., about A to 10% of hydrofluoric acid or equivalent amounts of fluoride salts, such as N-aF, NaBF,, etc. These pickling solutions are best used at temperatures between and 190 F. Other specific conditions follow:

Aqueous solutions containing from 15 to 50% HNO and 0.5 to 10% of fluoride ions, give a good finished surface and, as indicated in the attached figure, a minimum absorption of hydrogen. An aqueous pickling solution containing about 30% nitric acid and 4% hydrofluoric acid has been found to give exceedingly good results. It dissolves about 0.001" in a minute at F. and results in very little hydrogen evolution and absorption by the metal. Below a concentration of 0.005% H, damage to the metal is so slight as to be negligible. Thus, from the accompanying graph it is clear that 15% is the minimum HNO; content for safe operation. No reduction in hydrogen pickup is noted above 30% HNO Increases of HNO, beyond 50% are uneconomical. The

- Patented Dec. 20,- 1 96.0

. 3 most objectionable feature to HNO type pickling solution istheevolution of the poisonous N0, gas. This is especially hazardous wherever the ventilating facilities are not adequate. The major advantage is the prevention of hydrogen absorption and thus embrittlement.

Pickling solutions containing from 0.5 to fluoride ions and from 5 to 35% hydrogen peroxide (H 0 also impart 'good surface finish to the metal and result in little hydrogen pickup. The major advantage of this type of pickling solution is that no poisonous or otherwise objectionable gases are given off by the bath. The products of reaction are simply water and a titanium complex. "Optimum results are obtained with an aqueous bath containing about 20% H 0 and 2% fluoride ions. At 150 F. this bath pickles off about 0.001" of metal per minute. Lower temperatures are often preferred to minimize decomposition of the H 0 v Aqueous solutions containing from 2 to 50% HNO 0.5 to 30% H 0 and 0.5 to fluoride ions are especially attractive. Solutions of this type give excellent surfaces and very little hydrogen pickup, and are also the most economical and do not fume. Optimum results are obtained with such solutions containing about 20% HNO 2% H 0 and 2% fluoride ions. This concentration of HNO assures very little hydrogen pickup, and the H 0 prevents fuming by oxidizing the NO to N0 and thus to HNO before it leaves the pickling solutionas little as 0.5% H 0 i's'effective.

The advantages to be derived from hydrogen peroxide additions are: (1) the prevention of NO and thus NO; evolution to the air (N0 is not only obnoxious but poisonous; 0.01% has been fatal); (2) it results in little or noloss of nitric acid; (.3") it therefore eliminates the problem of nitric acid control; and (4) it eliminates staining of the metal sheet by NO; on withdrawing the sheet from the bath. The presence of a surplus of H 0 is indicated by the color, since H 0 added to a solution containing titanium salts imparts a color varying from yellow to deep red with increasing H 0 content. greatly simipli'fies' control. Modifications of the three types of pickling solutions mentioned embrace the use of other oxidants, including the aeration of the solution with air, oxygen or ozone, in order to minimize the escaping of NO and N0 The previously mentioned solutions can be modified to advantage by the addition of about 0.1% of a suitable wetting agent (epgl, Queene #5 by Quaker State, or Actival 57 by Haas Miller Corp.) A specific advantage is the prevention or vertical striations, which results from nonuniform pickling due to the rapid vertical movement of gas bubbles and solution along the sheet surface. The

This

use of a wetting agent increases the fuming from I-INOa-HF pickles but this is no problem if H 0 is employed in the solution or if the ventilation is adequate.

What is claimed is:

1. An aqueous solution for pickling and removing surface metal without embrit'tling the underlying surface of a metal selected-from the groupconsisting of titanium, zirconium, titanium base alloys and zirconium base alloys, containing in percentages by weight about 0.5 to 10% fluoride ions, about 2 to 50%: of nitric acid and about 0.5 to 30% of hydrogen peroxide.

2. The method of pickling a metal selected from the group consisting. of titanium, zirconium and alloys of each with rembval of metal-from the surface, which comprises: immersing the meal in an aqueous pickling solution containing in percentages by weight, about 0.5 to 10% of fluoride ions and at least one member selected from the group consisting of 5 to 35 hydrogen peroxide, and 2 to 50% nitric acid together with 0.5 to 30% hydrogen peroxide, while maintaining said solution at a temperature of about I00 to 190 F.

3. The method of pickling a metal selected from the group consisting of titanium, zirconium and alloys of each with removal of metal from the surface, which comprises: immersing the metal in an aqueous pickling solution containing in percentages by weight, about 0.5 to 5% of fluoride ions, and about 5 to 35% of hydrogen peroxide, while maintaining'said solution at a temperature of about to F.

4. The method of pickling a metal selected from the group consisting of titanium, zirconium and alloys of each with removal of metal from the surface, which com prises: immersing themetal in an aqueous pickling solution containing in percentages by weight, about 0.5 to 10% of fluoride ions, about 2 to 50% of nitric acid and about 0.5 to 30% of hydrogen peroxide while maintaining said solution at a temperature of about 100 to 190 F.

References Cited in the file of this patent UNITED STATES PATENTS France Oct. 19, 1955 

3. THE METHOD OF PICKLING A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND ALLOYS OF EACH REMOVAL OF METAL FROM THE SURFACE, WHICH COMPRISES: IMMERSING THE METAL IN AN AQUEOUS PICKLING SOLUTION CONTAINING IN PERCENTAGES BY WEIGHT, ABOVE 0.5 TO 5% OF FLUORIDE IONS, AND ABOUT 5 TO 35% OF HYDROGEN PEROXIDE, WHILE MAINTAINING SAID SOLUTION AT A TEMPERATURE OF ABOUT 100* F. TO 190* F. 